Isobutene is the raw material to manufacture alkylate, MTBE, diisobutylene, polyisobutene, methacrylic acid, butylated phenols, etc. There are three sources of isobutene: catalytic cracking, steam cracking, and dehydrogenation of butanes. However, in those processes isobutene is always produced as a mixture of C4s. It is costly to produce high purity isobutene from those mixtures due to their close boiling points.
There are currently three important processes for the production of high purity isobutene: (1) the extraction process using an acid to separate isobutene, (2) the dehydration of tert-butanol, and (3) the cracking of MTBE. The expected demand for MTBE precludes the third route for isobutene production. MTBE is more valuable than TBA due to its huge demand as an additive in gasoline fuel. TBA will be an important source of isobutene and MTBE. Large quantities of TBA obtained as a by-product from propylene oxide (PO) plants become an important source for isobutene and MTBE.
U.S. Pat. No. 4,918,244 discloses a one-step process for preparing MTBE. TBA and methanol are continuously fed to a rectification tower having a packed solid-acid catalyst bed where the TBA and methanol react in the presence of the catalyst to produce MTBE. Stream data show that MTBE was the only product and isobutene was not produced in the claimed process. U.S. Pat. No. 4,925,989 discloses a process for preparing MTBE. TBA, isobutene and methanol are continuously fed into a combination reactor distillation tower having a packed sulfonic acid resin catalyst beds. However, a third raw material of isobutene is required to enhance MTBE yield in this invention. Although isobutene could be recovered from the product, but it eventually consumed and not produced. Consequently, isobutene is not co-produced in the disclosure of these two patents.
Integrating a prereactor and a CD column, the disclosed processes for MTBE production in U.S. Pat. Nos. 5,705,711 and 5,741,953 also indicate that isobutene is not co-produced. In the latter case, the isobutene is recovered as a mixture together with TBA and methanol, and recycled to the primary reactor to produce MTBE. Also, the CD column behind a prereactor is not charged with fresh TBA/methanol and is an auxiliary equipment to enhance MTBE yield. The molar ratio of methanol-to-TBA at the prereactor reaches 2.0 so that the energy consumption in recycling methanol is very high.
A serious of acid catalysts were disclosed for converting TBA to MTBE, such as ion-exchange resin, fluorosulfonic acid-treated Y-zeolite and acidic montmorillonite silica-alumina (U.S. Pat. Nos. 5,081,318 and 5,099,072). TBA and methanol were contacted with these catalysts and mostly first-stage converted to isobutene and MTBE. A fluoride-treated Y-zeolite or silicoaluminophosphate (U.S. Pat. Nos. 5,313,006 and 5,856,588) are claimed to have the ability to improve MTBE yield via the second-stage etherification of the unreacted or recovered TBA.
On the other hand, following patents are focus on the isobutene production via TBA dehydration. U.S. Pat. No. 4,423,271 discloses a method, which uses ion exchange resin as catalyst to dehydrate aqueous TBA in a fixed bed. U.S. Pat. No. 5,811,620 discloses a TBA dehydration method for producing isobutene via a reactive distillation column packed with fluoride-treated catalyst. A similar method is also disclosed in U.S. Pat. No. 5,849,971. However, no methanol is fed to the column and MTBE cannot be cogenerated with isobutene. In order to manufacture MTBE, an etherification process is required to follow with these TBA dehydration process.
The above methods are limited for industrial application. For example, TBA at high purity is solid state as temperature is below 25° C. The transportation for market trade is inconvenient. Also, these patented processes should be localized around the PO process or isobutene hydration process to avoid the TBA transportation problem.
The kinetics of liquid phase synthesis of MTBE from TBA and methanol catalyzed by ion exchange resin is proposed by Matouq and Goto, Int. J. Chem. Kinnet., 1993, 25, 825-831. In their report, the TBA and methanol undergo etherification and dehydration simultaneously over Amberlyst 15 such that MTBE and isobutene can be co-produced.